In nearly all forms of human cancer, the development of necrosis is tightly linked with malignant progression. Whether necrosis accelerates progression or is largely passive remains an open question, yet modeling these events to establish mechanisms and therapeutic vulnerabilities in animals has been challenging. In glioblastoma (GBM; WHO grade IV), the most malignant primary brain tumor, the rapid, radial growth phase that leads quickly to death is consistently preceded by the development of central necrosis. While genetic alterations of GBM are known in great detail, the biological properties that result from their acquisition and lead to this accelerated growth phase require deeper investigation. The tumor microenvironment (TME) changes dramatically following the onset of necrosis, from a sheet-like growth of infiltrating cells with relatively constant growth properties to a highly complex and evolving 3-D microsystem composed of diverse cell types and spatially segregated signaling networks. To better understand the dynamic temporal and spatial changes that promote progression, we propose to advance mouse models that closely parallel these events in human gliomas, since many mouse models of GBM lack necrosis. We developed a novel method to induce focal necrosis within high grade gliomas in vivo and will study TME restructuring and its impact on glioma growth in real time using multiphoton microscopy. As translational applications, we will demonstrate how hypoxia and necrosis promote the enrichment of glioma stem cells (GSCs) in their peri-necrotic niche and lead to the dramatic influx of tumor-associated macrophages (TAMs), which increase in number over 10-fold in the human disease. We propose both genetically characterized patient-derived GBM xenografts grown in mice with humanized immune cells, as well as an immunocompetent RCAS/tv-a model, and will determine how antagonizing these processes impact disease progression and outcomes. Our preliminary data and the literature indicate substantial differences between pre-necrotic and necrotic gliomas with regard to GSC and TAM enrichment and their impact on biological properties, but the mechanisms and evolution have not been studied in depth, in large part due to the absence of a credible animal model. Our model will capture glioma growth dynamics, GSC enrichment, and TAM influx, and facilitate the development of therapies that antagonize these mechanisms to improve outcomes.